Apparatus and method for measuring vibration characteristics

ABSTRACT

A vibration characteristic measuring apparatus includes a magnetic bearing that generates magnetic force on the rotating body of a multi-stage centrifugal compressor, a displacement sensor that measures the vibration amplitude of the rotating body, a current amplifier that supplies a current to the magnetic bearing, and an excitation controller which outputs an excitation control signal for applying vibration to the rotating body and which measures the response characteristics of the vibration of the rotating body to the excitation control signal. The excitation controller outputs a rotating body control signal obtained by adding a vibration eliminating signal for eliminating unbalance vibration of the rotating body to the excitation control signal when measuring the response characteristics, and the current amplifier supplies a current that generates magnetic force in accordance with the rotating body control signal to the magnetic bearing.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority of Japanese Application No.2011-101422, filed on Apr. 28, 2011, the entire specification, claimsand drawings of which are incorporated herewith by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vibration characteristic measuringapparatus that measures vibration characteristics of a rotational shaftof a rotating machine like a multi-stage centrifugal compressor and avibration characteristic measuring method.

2. Description of the Related Art

A multi-stage centrifugal compressor, which is installed in a petroleumfield, a natural gas plant, and a petrochemical plant, etc., and whichcompresses a compressible fluid, such as hydrocarbon or carbon dioxidegases, rotates centrifugal impellers of multi-stages along with arotational shaft, and compresses the compressible fluid by centrifugalforce.

Such a multi-stage centrifugal compressor is provided with a sealingmechanism that has an annular sealing member around the axial line ofthe rotational shaft so as to suppress the leak of the pressurized fluidfrom each stage of the centrifugal impeller.

According to the sealing mechanism of the multi-stage centrifugalcompressor, the instability factor due to the leak and flow of thepressurized fluid occurs, which affects the stability of the rotationalshaft and the centrifugal impeller. That is, when the instability factorexceeds the stability factor, self-induced vibration so-called sealwhirl occurs.

That is, it is important to design the rotating machine so as to havelarger stability sufficiently than instability generated at the sealingmechanism. In order to do so, it is necessary to measure the vibrationcharacteristics of a rotating body including the rotational shaft tofind out the characteristics thereof, and to evaluate the stabilitybased on the measured vibration characteristics.

For example, JP H05-5057 B discloses a technique of applying a vibrationto a bearing, and analyzing the vibration generated by a rotationalshaft due to the former vibration, thereby measuring the vibrationcharacteristics of a rotating machine in operation. Also, Pettinato, etal., “Shop Acceptance Testing of Compressor Rotordynamic Stability andTheoretical Correlation”, 39^(th) Turbo Machinery Symposium, Texas,2011, p.p. 31-42 discloses a technique of attaching an active radialmagnetic bearing to an end of a rotational shaft of a compressor,supplying an excitation current to the magnetic bearing to applyvibration to the rotational shaft, thereby measuring the vibrationcharacteristics of the rotational shaft.

The above-explained two techniques are to apply vibration to a rotatingrotational shaft, measure the vibration (an amplitude and a phase) ofthe rotational shaft, thereby measuring the vibration characteristics ofthe rotational shaft.

When the vibration characteristics of a rotating body like therotational shaft is measured in this manner, in order to obtainhigh-quality data with little variability, it is preferable that theamplitude of the applied vibration should be large. By increasing thevibration amplitude of the rotating body excited by the appliedvibration, the S/N ratio of a signal output from a sensor measuring thevibration is improved, and thus high-quality data indicating thevibration characteristics can be obtained.

On the other hand, when the vibration amplitude of the rotating body isincreased by the applied vibration, if it exceeds an allowable value setas a designed value, for example, the rotating body contacts the sealingmechanism, and the sealing member of the sealing mechanism is worn out.Hence, it is necessary to apply vibration within a range where thevibration amplitude of the rotating body does not exceed the allowablevalue.

However, the rotating body of the rotating machine is vibrated within arange where the vibration amplitude does not exceed the allowable valuein the normal rotation due to an unbalanced weight, etc. Accordingly,the amplitude of the vibration that can be excited by applying vibrationto the rotating body is restricted within the range of a margin from theamplitude of the vibration at the non-excited condition to the allowablevalue. In other words, the vibration excited by the applied vibration atthe rotational shaft is restricted and small, and thus the S/N ratio ofa signal output from the sensor measuring the vibration becomes poor.Hence, the quality of data (data indicating the vibrationcharacteristics of the rotational shaft) obtained based on such a signaldeteriorates.

For example, when the quality of obtained data is poor, vibrationanalysis can be made by largely increasing the number of data to beobtained and averaging those pieces of data.

However, when the large numbers of data are obtained for averaging, ittakes a long time to obtain the large numbers of data and the number ofprocess steps is increased. Also, the operation time of the rotatingmachine and the time for applying vibration become long in order toobtain the large numbers of data, resulting in the increase of energyconsumption.

Therefore, it is an object of the present invention to provide anapparatus and a method for measuring vibration characteristics which canvibrate a rotating body without exceeding an allowable value whenmeasuring vibration characteristics, and which can obtain high-qualitydata without increasing energy consumption.

SUMMARY OF THE INVENTION

In order to achieve the above object, an aspect of the present inventionprovides an apparatus for measuring vibration characteristics,including: a magnetic bearing that generates a magnetic force on arotating body of a rotating machine in a non-contact manner; a measuringdevice that measures an vibration amplitude when the rotating body isvibrated; a current supplier that supplies a current to the magneticbearing; and an excitation controller which outputs an excitationcontrol signal for controlling the magnetic bearing to apply vibrationto the rotating body, and which measures a response characteristics ofthe vibration of the rotating body to the excitation control signalbased on the vibration measured by the measuring device. The excitationcontroller outputs a rotating body control signal obtained by adding avibration eliminating signal to the excitation control signal for themagnetic bearing to eliminate unbalance vibration generated when therotating body rotates when the response characteristics is measured, andthe current supplier supplies a current that generates magnetic force inaccordance with the rotating body control signal to the magneticbearing. Also, the present invention provides a method for measuringvibration characteristics by the above-explained vibrationcharacteristic measuring apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic configuration diagram showing a multi-stagecentrifugal compressor provided with a vibration characteristicmeasuring apparatus according to an embodiment of the present inventionas viewed from a side;

FIG. 1B is a schematic view showing a magnetic bearing of the vibrationcharacteristic measuring apparatus and a rotational shaft of themulti-stage centrifugal compressor as viewed in a direction A;

FIG. 2A is a diagram modeling unbalance vibration of the rotationalshaft;

FIG. 2B is a diagram for explaining a margin from a vibration amplitudeof the rotational shaft to an allowable value;

FIG. 3 is a diagram showing functional blocks of the vibrationcharacteristic measuring apparatus;

FIG. 4A is a diagram showing a vibration amplitude in an X-axisdirection of a rotational shaft when a vibration is applied by aconventional vibration characteristic measuring apparatus;

FIG. 4B is a diagram showing a vibration amplitude in an X-axisdirection of a rotational shaft when a vibration is applied by thevibration characteristic measuring apparatus of this embodiment;

FIG. 5 is a diagram for comparing a margin from a vibration amplitude ofa rotational shaft to an allowable value of this embodiment with that ofa prior art; and

FIG. 6 is a functional block diagram of a vibration characteristicmeasuring apparatus provided with an applied-vibration control devicehaving an applied-vibration signal breaker.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be explained in detail withreference to the accompanying drawings.

As shown in FIG. 1A, a vibration characteristic measuring apparatus 1 ofthis embodiment is attached to a rotating machine like a multi-stagecentrifugal compressor 50 and is capable of measuring vibrationcharacteristics of a rotating body including a rotational shaft 51, acentrifugal impeller 53, etc. of the multi-stage centrifugal compressor50.

The vibration characteristic measuring apparatus 1 of this embodimentcan be applied to not only the multi-stage centrifugal compressor 50 butalso a rotating machine (e.g., a turbine, or a motor) having a rotatingbody like the rotational shaft 51, but the following explanation will begiven to an example case in which the vibration characteristic measuringapparatus 1 is attached to the multi-stage centrifugal compressor 50.

The multi-stage centrifugal compressor 50 employing the configurationshown in FIG. 1A successively compresses a compressible fluid like a gasby the centrifugal force of the centrifugal impeller 53 rotatingtogether with the rotational shaft 51, and the rotational shaft 51 isrotated and driven by a drive source (e.g., a motor) (not shown).

The centrifugal impeller 53 is provided in the axial direction of therotational shaft 51 in a multi-stage manner, rotates together with therotational shaft 51, compresses the fluid suctioned from the center (ata side of the rotational shaft 51) and discharges the pressurized fluidfrom the outer periphery by the centrifugal force.

The rotational shaft 51 is supported by a plurality of bearing membersin a freely rotatable manner. For example, the rotational shaft 51 issupported in the radial direction by bearing members 52 a and 52 bprovided at two locations so as to hold the centrifugal impeller 53therebetween in the axial direction. Also, a bearing member 52 c thatsupports the rotational shaft 51 in the axial direction restricts thedisplacement of the rotational shaft 51 in the axial direction.

The rotational shaft 51 is supported by the plurality of bearing members52 a to 52 c in a freely rotatable manner in this fashion.

The centrifugal impeller 53 is housed in a housing 54 that is a staticbody, and the centrifugal impeller 53 provided in the multi-stage mannercompresses the fluid taken in from an inlet 54 a and discharges thecompressed fluid from an outlet 54 b.

Hereinafter, a structure which includes the rotational shaft 51 and thecentrifugal impeller 53, and which rotates together with the rotationalshaft 51 is collectively referred to as a “rotating body” in some cases.Also, a structure (e.g., the housing 54) that stands still against therotating body is collectively referred to as a “static body” in somecases.

According to the multi-stage centrifugal compressor 50 employing theabove-explained configuration, when the fluid taken in from the inlet 54a of the housing 54 is discharged from the housing 54 without passingthrough the centrifugal impeller 53, the pressurized fluid is dischargedfrom the outlet 54 b in an uncompressed condition, and thus thecompression efficiency of the multi-stage centrifugal compressor 50decreases. Hence, in order to improve the compression efficiency, asealing mechanism 55 that seals a space between the housing 54 (thestatic body) and the rotational shaft 51, the centrifugal impeller 53(the rotating body) is provided as needed.

The sealing mechanism 55 is provided so as to seal the space between thehousing 54 and the centrifugal impeller 53 in such a manner so as toprevent the pressurized fluid from, for example, flowing through thespace between the centrifugal impeller 53 and the housing 54 from theinlet (the inlet-54 a side) of the centrifugal impeller 53 to the outlet(the outlet-54 b side) thereof.

Sealing mechanisms 55 that seal respective spaces between the housing 54and the rotational shaft 51 are provided at both ends of the centrifugalimpeller 53 in the axial direction.

The sealing mechanisms 55 provided as needed include a sealing member(not shown) that forms a minute clearance between the static body andthe rotating body, and suppresses a flow of the pressurized fluid.

Also, by forming such a clearance, for example, the sealing member ofthe sealing mechanism 55 of the static body can be prevented fromcontacting the rotating body, and thus the worn-out of the sealingmember is suppressed.

Note that FIG. 1 shows a configuration that the static body (the housing54) has the sealing mechanisms 55, but the rotating body (the rotationalshaft 51 and the centrifugal impeller 53, etc.) may have the sealingmechanisms 55. Also, the static body other than the housing 54 may havethe sealing mechanisms 55.

The rotating body including the rotational shaft 51 and the centrifugalimpeller 53, etc., has an uneven mass in the circumferential directionand produces minute vibration (unbalance vibration) at the time ofrotating.

When the mass of the rotating body is uneven in the circumferentialdirection, as shown in FIG. 2A, the rotating body can be modeled so thata point in the circumferential direction becomes the concentrated pointof the mass (a mass concentrated point G). When the rotating body havingthe mass concentrated point G rotates, the centrifugal force acts on themass concentrated point G and the rotating body is pulled outwardly, andthus the rotating body displaces toward the mass concentrated point G.Such displacement successively occurs together with the rotation of therotating body, and thus unbalance vibration occurs.

Magnets 12X, 12Y (see FIG. 2A) of the magnetic bearing 12 of thevibration characteristic measuring apparatus 1 (see FIG. 1A) areprovided so as to generate a magnetic force on the rotational shaft 51in a non-contact manner.

As explained above, since the rotating body produces unbalancevibration, the sealing mechanism 55 of the static body preferably has aclearance so as not to contact the rotating body when such unbalancevibration occurs. However, when the clearance becomes large, the sealingperformance that suppresses the flow of the pressurized fluid decreases.

Hence, the unbalance vibration of the rotating body is always monitoredand the operation of the rotating body is managed in some cases so thatthe amplitude of such unbalance vibration does not exceed apredetermined allowable value.

According to a rotating machine like the multi-stage centrifugalcompressor 50 (see FIG. 1A), in order to evaluate the stability of therotating body relative to the characteristic vibration mode, it isnecessary to measure the vibration characteristics for measuring theresponse character of the lateral vibration produced at the rotatingbody as vibration characteristics.

The stability of the rotating body relative to the lateral vibration canbe evaluated by applying vibration to the rotating body, and measuringthe response characteristics of the vibration of the rotating bodyproduced by the application of the vibration.

Hence, to evaluate the stability of the rotating body relative to thelateral vibration, the rotating body is forcibly vibrated, i.e.,vibration is applied thereto, an amplitude and a phase of the vibrationof the rotating body at this time are measured, and the responsecharacteristics of an output (the actual vibration of the rotating body)relative to an input (the vibration applied to the rotating body) ismeasured.

As shown in FIG. 1A, the vibration characteristic measuring apparatus 1according to this embodiment can be attached to the multi-stagecentrifugal compressor 50, applies vibration to the rotating body inorder to evaluate the stability of the rotating body relative to thelateral vibration, and measures the vibration (the amplitude and thephase) of the rotating body to which the vibration is applied, therebymeasuring the response characteristics of the vibration.

Hence, the vibration characteristic measuring apparatus 1 includes themagnetic bearing 12 that applies vibration to the rotational shaft 51and a measuring device (a displacement sensor 13) that measures thevibration amplitude (a displacement) of the rotational shaft 51.

The magnetic bearing 12 includes the magnets 12X, 12Y (see FIG. 1B), andhas the primary function as a bearing to support the rotational shaft 51in a non-contact manner by magnetic force generated by a currentsupplied to coils (not shown).

As shown in FIG. 1B, a Y axis is set in the vertical direction (a seconddirection) on a plane having the axial line of the rotational shaft 51as a normal line and an X axis is set in a horizontal direction (a firstdirection) orthogonal to the Y axis. According to this setting, theX-axis direction (the first direction) is a direction orthogonal to theaxial line direction of the rotational shaft 51 (the rotating body), andthe Y-axis direction (the second direction) is a direction orthogonal tothe axial line direction of the rotational shaft 51 and the X-axisdirection (the first direction).

As shown in FIG. 1B, in a coordinate system having the rotational centerof the rotational shaft 51 as an origin O, the magnetic bearing 12 hasthe magnets 12X disposed so as to hold the rotational shaft 51therebetween in the horizontal direction over the X axis, and has themagnets 12Y disposed so as to hold the rotational shaft 51 therebetweenin the vertical direction over the Y axis. The gap between therotational shaft 51 and the magnet 12X can be adjusted by controlling acurrent supplied to the magnets 12X, and the gap between the rotationalshaft 51 and the magnet 12Y can be adjusted by controlling a currentsupplied to the magnets 12Y.

Furthermore, by successively adjusting the gap between the rotationalshaft 51 and the magnet 12X, the rotational shaft 51 can be vibrated inthe X-axis direction, and by successively adjusting the gap between therotational shaft 51 and the magnet 12Y, the rotational shaft 51 can bevibrated in the Y-axis direction. The magnetic bearing 12 can applyvibration to the rotational shaft 51 in this manner.

For example, when a cosine-wave current is supplied to the magnets 12Xso as to periodically adjust the gap between the rotational shaft 51 andthe magnet 12X, and a sine-wave current is supplied to the magnets 12Yso that the gap between the rotational shaft 51 and the magnet 12Y isadjusted to have the same period but the shifted phase by 90 degrees tothe gap between the rotational shaft 51 and the magnet 12X, vibrationcan be applied to the rotational shaft 51 so as to vibrate in a circularmotion around the axial line.

The displacement sensor 13 shown in FIG. 1A includes, for example, an Xdisplacement sensor 13X that measures a displacement of the rotationalshaft 51 in the X-axis direction caused by vibration and a Ydisplacement sensor 13Y that measures a displacement in the Y-axisdirection, and the vibration displacement of the rotational shaft 51 canbe measured based on the displacement in the X-axis direction and thedisplacement in the Y-axis direction. The kind of the displacementsensor 13 is not limited to any particular one, but an eddy current typethat can measure the displacement of the rotational shaft 51 in, forexample, a non-contact manner is available.

Conventionally, the vibration characteristic measuring apparatus 1 isconfigured to apply vibration to the rotational shaft 51 through themagnetic bearing 12 when the rotating body is, measure the displacementin the X-axis direction of the rotational shaft 51 and the displacementthereof in the Y-axis direction through the displacement sensor 13, andmeasure the vibration displacement of the rotational shaft 51.

At this time, since the rotating body is, the above-explained unbalancevibration occurs. Hence, when vibration is applied to the rotationalshaft 51, in addition to the unbalance vibration, the vibration(hereinafter, referred to as forced vibration) due to the application ofthe vibration occurs, and the amplitude of the rotating body increases.

As explained above, in order to suppress a contact between the sealingmechanisms 55 of the static body and the rotating body, it is necessarythat the vibration amplitude of the rotating body should be smaller thanthe predetermined allowable value. Hence, a vibration amplitudecorresponding to the margin from the amplitude of the unbalancevibration to the allowable value can be input to the rotating body atwhich the unbalance vibration occurs.

In other words, the magnitude of the forced vibration by application ofvibration is restricted. That is, as shown in FIG. 2B, with reference tothe amplitude of an unbalance vibration V1, it is necessary to suppressthe amplitude of a forced vibration V2 within a range of a margin (afirst margin) from the reference vibration amplitude to the allowablevalue.

Hence, the vibration amplitude of the rotational shaft 51 caused by theforced vibration becomes small, and the S/N ratio of the signal (adisplacement signal) which indicates the displacement of the rotationalshaft 51 and is output from the displacement sensor 13 becomes poor.

Therefore, the vibration characteristic measuring apparatus 1 of thisembodiment applies vibration with a large amplitude to the rotationalshaft 51, and attempts to maintain the good S/N ratio of thedisplacement signal output from the displacement sensor 13.

More specifically, the vibration characteristic measuring apparatus 1includes a balancing signal generator 8 as shown in FIG. 3, and attemptsto temporarily eliminate the unbalance vibration at the time ofmeasuring the response characteristics of the rotating body.

As shown in FIG. 2A, the balancing signal generator 8 successivelygenerates forces for cancelling the centrifugal force acting on the massconcentrated point G by the magnetic force caused by the magneticbearing 12 to cancel the displacement toward the mass concentrated pointG caused by the centrifugal force, thereby eliminating the unbalancevibration of the rotating body.

As explained above, the vibration characteristic measuring apparatus 1of this embodiment has a function of causing the balancing signalgenerator 8 shown in FIG. 3 to generate a signal for eliminating theunbalance vibration of the rotating body, and of inputting such a signalinto a current amplifier 11 (see FIG. 3) that is a current supplier ofthis embodiment. The current amplifier 11 amplifies the input signal(the signal for eliminating the unbalance vibration) at a predeterminedgain, and supplies the amplified signal to the magnetic bearing 12.

It is preferable that the current supplied to the magnetic bearing 12from the current amplifier 11 should be a current having the inputsignal amplified by the predetermined gain and be synchronized with theinput signal. According to this configuration, the magnetic bearing 12to which the current is supplied from the current amplifier 11 cangenerate magnetic force in accordance with the signal input to thecurrent amplifier 11.

The current supplier is not limited to the current amplifier 11 thatsupplies the current obtained by amplifying the input signal to themagnetic bearing 12 as long as it can supply a current for generatingmagnetic force in accordance with the input signal to the magneticbearing 12.

The balancing signal generator 8 shown in FIG. 3 includes a so-calledtwo-phase oscillator, and the oscillation frequency thereof needs to besynchronized with the rotation frequency of the rotating body. Hence, arotation pulse signal Rp indicating the rotating speed of the rotatingbody is input into the balancing signal generator 8. For example, aconfiguration may be employed in which a rotating speed sensor (notshown) inputs, as the rotation pulse signal Rp, a result of measuring arotation reference groove cut in the rotational shaft 51 to thebalancing signal generator 8.

Also, the balancing signal generator 8 is built with a phase locked loopcircuit (a PLL circuit) having the input rotation pulse signal Rp as areference signal, and is configured to oscillate a sine wave signal anda cosine wave signal synchronized with the rotation frequency of therotating body. The sine wave and the cosine wave oscillated by the PLLcircuit are single frequency signals having phases which are differentby 90 degrees each other.

Also, as shown in FIG. 3, a cancelling amplitude A and a phase Ph of theunbalance vibration are input into the balancing signal generator 8. Thephase Ph is a phase between the rotation angle of the rotating body androtation angle of the mass concentrated point G shown in FIG. 2A shiftedby 180 degrees. That is, it is an angle (indicated by θb in FIG. 2A)delayed by 180 degrees from the rotation angle (indicated by θa in FIG.2A) of the mass concentrated point G when the rotating body is at areference position (e.g., rotation angle “0” degrees).

By setting the phase Ph delayed by 180 degrees from the rotation angleof the mass concentrated point G, it becomes possible to generate forcein a direction for cancelling the centrifugal force acting on the massconcentrated point G.

Also, the cancelling amplitude Am indicates the level of cancelling thecentrifugal force acting on the mass concentrated point G.

The cancelling amplitude Am and phase Ph can be obtained by, forexample, an operator of the vibration characteristic measuring apparatus1 (see FIG. 1A), who measures the unbalance vibration caused by therotating body of the multi-stage centrifugal compressor 50 (see FIG.1A), and can be input through an input device (not shown) as numericvalues. Next, when a configuration is employed in which a memory unit(not shown) stores the input cancelling amplitude Am and phase Ph, thebalancing signal generator 8 can read the cancelling amplitude Am andphase Ph as needed to use those pieces of data.

The balancing signal generator 8 combines the cancelling amplitude Amand phase Ph with the cosine wave signal and the sine wave signaloscillated by the PLL circuit, and generates, as indicated by thefollowing formulae (1a) and (1b), an oscillation signal Fx in the X-axisdirection and an oscillation signal Fy in the Y-axis direction as shownin FIG. 2A.

Fx=Am·Cos(Ωt+Ph)  (1a)

Fy=Am·Sin(Ωt+Ph)  (1b)

where Ω is the rotation angle speed of the rotating body and t is atime.

Also, the rotation angle speed Ω is a value calculated based on, forexample, the input rotation pulse signal Rp.

The balancing signal generator 8 generates the oscillation signals Fxand Fy both having the amplitudes Am but the phases shifted by 90degrees from each other in this manner.

In this fashion, the balancing signal generator 8 generates theoscillation signal Fx in the X-axis direction containing the cosine wavecomponent (Am·Cos(Ωt+Ph)) and the oscillation signal Fy in the Y-axisdirection containing the sine wave component (Am·Sin(Ωt+Ph)).

The oscillation signals Fx and Fy indicated by the formulae (1a) and(1b) are output from the balancing signal generator 8, and are inputinto an X amplifier 11X and a Y amplifier 11Y, respectively, of thecurrent amplifier 11.

The X amplifier 11X amplifies the input oscillation signal Fx at apredetermined gain, and supplies the amplified signal to the magnets12X. Also, the Y amplifier 11Y amplifies the input oscillation signal Fyat a predetermined gain (e.g., the same gain as that of the X amplifier11X), and supplies the amplified signal to the magnets 12Y. The gains ofthe X amplifier 11X and the Y amplifier 11Y are preferably set dependingon the characteristics of the magnetic bearing 12 and the vibrationcharacteristics of the rotating body, respectively.

When the currents output from the current amplifier 11 (the X amplifier11X and the Y amplifier 11Y) are supplied to the magnets 12X and themagnets 12Y, the currents having a signal (a synthesized oscillationsignal Fz) synthesized in the X-axis direction and the Y-axis directionand amplified are supplied to the magnetic bearing 12. The synthesizedoscillation signal Fz can be expressed by the following formula (2)using the complex number representation.

Fz=Fx+jFy=Am·e ^(j(Ωt+Ph))  (2)

where j is an imaginary number unit.

It is clear from the formula (2) that the synthesized oscillation signalFz is a signal oscillating circularly with a radius of Am over a complexplane together with the advancement of the time t. Hence, when themagnets 12X and 12Y disposed around the rotating body by the magneticbearing 12 and orthogonal to one another generate respective magneticforces in accordance with the oscillation signals Fx and Fy, thesynthesized magnetic force is synchronized with the rotation of therotating body and oscillates circularly around the rotation center ofthe rotating body.

Also, the force (hereinafter, referred to as unbalance force), which isgenerated by the unbalance vibration and displaces the rotating body,also oscillates circularly around the rotation center of the rotatingbody in synchronization with the rotation of the rotating body. Theunbalance force is generated by the centrifugal force acting on the massconcentrated point G shown in FIG. 2A, and the phases Ph of theoscillation signals Fx and Fy correspond to an angle delayed by 180degrees from the rotation angle of the mass concentrated point G.Accordingly, the unbalance vibration of the rotating body can beeliminated by the magnetic force generated by the magnetic bearing 12 towhich the cosine wave current and the sine wave current generated byamplifying the oscillation signals Fx and Fy are supplied.

That is, the synthesized oscillation signal Fz is a vibrationeliminating signal containing the cosine wave component (Am·Cos(Ωt+Ph))as a component for eliminating the vibration in the X-axis direction andthe sine wave component (Am·Sin(Ωt+Ph)) as a component for eliminatingthe vibration in the Y-axis direction.

The balancing signal generator 8 generates the oscillation signals Fxand Fy in this manner, and further generates the synthesized oscillationsignal (the vibration eliminating signal) Fz by synthesizing theoscillation signals Fx and Fy, thereby eliminating the unbalancevibration of the rotating body.

Also, the vibration characteristic measuring apparatus 1 includes anexcitation response analyzer 9 shown in FIG. 3. The excitation responseanalyzer 9 generates excitation signals Ex and Ey for applyingvibrations to the rotating body. The excitation signal Ex is a signal toapply vibration to the rotating body in the X-axis direction, and theexcitation signal Ey is a signal to apply vibration to the rotating bodyin the Y-axis direction. A signal containing at least one of theexcitation signals Ex and Ey as a component is referred to as anexcitation control signal.

According to this configuration, the excitation signal Ex is a component(a first direction excitation component) of the excitation controlsignal that applies vibration to the rotating body in the X-axisdirection, and the excitation signal Ey is a component (a seconddirection excitation component) of the excitation control signal thatapplies vibration to the rotating body in the Y-axis direction.

The excitation response analyzer 9 of this embodiment outputs, forexample, sine wave signals with arbitrary amplitudes as the excitationsignal Ex in the X-axis direction and the excitation signal Ey in theY-axis direction while changing the periods, and inputs those signalsinto the X amplifier 11X and the Y amplifier 11Y of the currentamplifier 11, respectively.

When a current obtained by amplifying the excitation signal Ex that isthe sine wave signal by the X amplifier 11X is supplied to the magnets12X, vibration is applied to the rotating body in the X-axis direction.Also, when a current obtained by amplifying the excitation signal Eythat is the sine wave signal by the Y amplifier 11Y is supplied to themagnets 12Y, vibration is applied to the rotating body in the Y-axisdirection.

Also, a displacement signal indicating the displacement of the rotatingbody measured by the displacement sensor 13 is input into the excitationresponse analyzer 9. The X displacement sensor 13X measures thedisplacement of the rotating body in the X-axis direction, and inputs,as an X displacement signal Mx, the measurement result into theexcitation response analyzer 9, and the Y displacement sensor 13Ymeasures the displacement of the rotating body in the Y-axis directionand inputs, as a Y displacement signal My, the measurement result intothe excitation response analyzer 9.

When the excitation signal Ex in the X-axis direction is a sine wavesignal, the excitation response analyzer 9 outputs the excitation signalEx while changing the period along with the advancement of the time, andobtains data on the displacement of the rotating body in the X-axisdirection based on the X displacement signal Mx input into theexcitation response analyzer 9. Thereafter, the response characteristics(the frequency characteristics) of the rotating body in the X-axisdirection relative to the excitation signal Ex (the sine wave signal) ismeasured.

Likewise, when the excitation signal Ey in the Y-axis direction is asine wave signal, the excitation response analyzer 9 outputs theexcitation signal Ey while changing the period along with theadvancement of the time, and obtains data on the displacement of therotating body in the Y-axis direction based on the Y displacement signalMy input into the excitation response analyzer 9. Thereafter, theresponse characteristics (the frequency characteristics) of the rotatingbody in the Y-axis direction relative to the excitation signal Ey (thesine wave signal) is measured.

In this fashion, the excitation response analyzer 9 measures theresponse characteristics of the output (the X displacement signal Mx andthe Y displacement signal My) of the rotating body relative to the input(the excitation signal Ex in the X-axis direction and the excitationsignal Ey in the Y-axis direction) to the rotating body.

For example, as shown in FIG. 4A, in order to measure the responsecharacteristics in the X-axis direction, when the excitation responseanalyzer 9 applies vibration to the rotating body in the X-axisdirection, while vibration is applied (during an excitation period), thedisplacement in the X-axis direction, i.e., the amplitude changes inaccordance with the change in the period of the excitation signal Ex inthe X-axis direction, and the response characteristics which becomes themaximum amplitude at a specific frequency can be obtained.

However, since the unbalance vibration is generated by the rotating bodywhile vibration is applied, the amplitude of the forced vibration causedby the excitation is superimposed on the amplitude of the unbalancevibration. Also, the vibration amplitude of the rotating body subjectedto the forced vibration becomes larger than that of the rotating body towhich no vibration is applied. Hence, the vibration amplitude that canbe produced at the rotating body by the excitation is restricted withinthe range of the first margin from the amplitude of the unbalancevibration to the allowable value as shown in FIG. 2B (again shown inFIG. 5 as (conventional example)).

That is, as is shown by (conventional example) in FIG. 5, with referenceto the amplitude of an unbalance vibration V1, the amplitude of a forcedvibration V2 appears as the increase from the reference vibrationamplitude. Hence, the amplitude of the forced vibration V2 is limited tothe amplitude corresponding to the first margin from the referencevibration amplitude (the amplitude of the unbalance vibration V1) to theallowable value. This restricts the vibration amplitude generated byapplying vibration to the rotating body to be small. That is, it islimited in such a way that the maximum amplitude of the forced vibrationV2 becomes small and the S/N ratio of the X displacement signal Mxoutput from the X displacement sensor 13X measuring the maximumamplitude becomes poor. Accordingly, the quality of the data on thedisplacement in the X-axis direction obtained by the excitation responseanalyzer 9 becomes poor. The quality of data on the displacement in theY-axis direction becomes also poor.

Hence, the vibration characteristic measuring apparatus 1 (see FIG. 1A)includes, as shown in FIG. 3, an adder 7 a which adds the oscillationsignal Fx to the excitation signal Ex, and adds the oscillation signalFy to the excitation signal Ey. According to this embodiment, thebalancing signal generator 8, the excitation response analyzer 9, andthe adder 7 a function are included as an excitation controller 7 of thevibration characteristic measuring apparatus 1.

When measuring the response characteristics of the rotating body, thevibration characteristic measuring apparatus 1 causes the excitationresponse analyzer 9 of the excitation controller 7 to generate theexcitation control signal, and causes the balancing signal generator 8to generate the vibration eliminating signal. Also, the adder 7 agenerates an addition signal ADDx obtained by adding the oscillationsignal Fx in the X-axis direction of the vibration eliminating signal tothe excitation signal Ex in the X-axis direction of the excitationcontrol signal. Likewise, the adder 7 a generates an addition signalADDy obtained by adding the oscillation signal Fy in the Y-axisdirection of the vibration eliminating signal to the excitation signalEy in the Y-axis direction of the excitation control signal.

The addition signal ADDx is input into the X amplifier 11X, and acurrent that is the addition signal ADDx amplified by the X amplifier11X is supplied to the magnets 12X (see FIG. 1B) of the magnetic bearing12.

Also, the addition signal ADDy is input into the Y amplifier 11Y, and acurrent that is the addition signal ADDy amplified by the Y amplifier11Y is supplied to the magnets 12Y (see FIG. 1B) of the magnetic bearing12.

That is, the excitation controller 7 outputs a signal (hereinafter,referred to as a rotating body control signal) obtained by adding thevibration eliminating signal (the synthesized oscillation signal Fz) tothe excitation control signal.

The rotating body control signal is a synthesized signal containing theaddition signal ADDx which contains a component obtained by adding thecosine wave component of the vibration eliminating signal to the firstdirection excitation component of the excitation control signal, and theaddition signal ADDy which contains a component obtained by adding thesine wave component of the vibration eliminating signal to the seconddirection excitation component of the excitation control signal.

The addition signal ADDx is obtained by adding the oscillation signal Fxin the X-axis direction to the excitation signal Ex, and contains thecomponent of the oscillation signal Fx in the X-axis direction of therotating body, and the component of the excitation signal Ex in theX-axis direction.

Also, the addition signal ADDy is obtained by adding the oscillationsignal Fy in the Y-axis direction to the excitation signal Ey, andcontains the component of the oscillation signal Fy in the Y-axisdirection of the rotating body and the component of the excitationsignal Ey in the Y-axis direction.

Hence, the rotating body control signal containing the addition signalsADDx and ADDy as components is a signal containing a component of theoscillation signal Fx in the X-axis direction, a component of theoscillation signal Fy in the Y-axis direction, a component of theexcitation signal Ex in the X-axis direction, and a component of theexcitation signal Ey in the Y-axis direction. When a current having therotating body control signal amplified by the current amplifier 11 issupplied to the magnetic bearing 12, a current containing the componentof the oscillation signal Fx in the X-axis direction, the component ofthe oscillation signal Fy in the Y-axis direction, the component of theexcitation signal Ex in the X-axis direction, and the component of theexcitation signal Ey in the Y-axis direction is supplied to the magneticbearing 12.

According to this configuration, during an excitation period, theunbalance vibration of the rotating body is eliminated by the componentsof the oscillation signals Fx and Fy in the current supplied to themagnetic bearing 12, and vibrations are applied to the rotating body inthe X-axis direction and the Y-axis direction by the components of theexcitation signals Ex and Ey. Hence, as shown in FIG. 4B, no unbalancevibration occurs in the X-axis direction during the excitation period,and only the forced vibration by caused the application of vibrationsoccurs. The vibration amplitude of the rotating body in the X-axisdirection becomes that of the forced vibration. Also, it is notillustrated in the figure but no unbalance vibration in the Y-axisdirection occurs, and only the forced vibration caused by theapplication of vibrations occurs. The vibration amplitude of therotating body in the Y-axis direction becomes that of the forcedvibration.

Hence, the X displacement sensor 13X (see FIG. 3) measures only theamplitude of the forced vibration in the X-axis direction, and inputs,as the X displacement signal Mx (see FIG. 3), the measurement resultinto the excitation response analyzer 9 (see FIG. 3). Likewise, the Ydisplacement sensor 13Y (see FIG. 3) measures only the amplitude of theforced vibration in the Y-axis direction, and inputs, as the Ydisplacement signal My (see FIG. 3), the measurement result into theexcitation response analyzer 9 (see FIG. 3).

That is, as is indicated as “embodiment” in FIG. 5, a condition in whichno amplitude of the unbalance vibration is present, i.e., a conditionthat the amplitude is “0” can be a reference, and the amplitude of theforced vibration V2 appears as the increase from “0”. Hence, it isappropriate if the amplitude of the forced vibration V2 is within arange of an amplitude corresponding to a margin (the second margin) from“0” to the allowable value, and thus the range of the vibrationamplitude generated by applying vibration to the rotating body can beextended. Also, the maximum amplitude of the forced vibration V2 can beextended up to a vibration amplitude corresponding to the second marginfrom “0” to the allowable value, and thus the X displacement sensor 13X(see FIG. 3) measuring the maximum amplitude can output the Xdisplacement signal Mx (see FIG. 3) with a good S/N ratio. Likewise, theY displacement sensor 13Y (see FIG. 3) measuring the maximum amplitudecan output the Y displacement signal My (see FIG. 3) with a good S/Nratio.

Accordingly, the quality of the data on the displacement in the X-axisdirection and that of data on the displacement in the Y-axis directionboth obtained by the excitation response analyzer 9 can be improved.

The vibration characteristic measuring apparatus 1 can employ aconfiguration of measuring independently the response characteristics ofthe rotating body of the multi-stage centrifugal compressor 50 (see FIG.1A) in the X-axis direction and the response characteristics in theY-axis direction.

For example, when the response characteristics in the X-axis directionis independently measured, the excitation response analyzer 9 inputs theaddition signal ADDx obtained by adding the oscillation signal Fx in theX-axis direction output from the balancing signal generator 8 and theexcitation signal Ex in the X-axis direction output from the excitationresponse analyzer 9 to the X amplifier 11X.

Also, the oscillation signal Fy in the Y-axis direction output from thebalancing signal generator 8 is input into the Y amplifier 11Y. That is,the addition signal ADDy that nulls the excitation signal Ey in theY-axis direction is input into the Y amplifier 11Y.

At this time, a current containing the component of the oscillationsignal Fx in the X-axis direction, the component of the oscillationsignal Fy in the Y-axis direction, and the component of the excitationsignal Ex in the X-axis direction is supplied to the magnetic bearing12.

Next, the X displacement sensor 13X (see FIG. 3) measures thedisplacement of the rotating body in the X-axis direction, and inputsthe X displacement signal Mx (see FIG. 3) into the excitation responseanalyzer 9 (see FIG. 3).

The rotating body has the unbalance vibration in the X-axis directioneliminated by the component of the oscillation signal Fx in the X-axisdirection in the current supplied to the magnetic bearing 12, and the Xdisplacement sensor 13X becomes able to measure the vibration amplitudeof the rotating body vibrated by the application of vibration.

Likewise, the excitation response analyzer 9 inputs the addition signalADDy obtained by adding the oscillation signal Fy in the Y-axisdirection output from the balancing signal generator 8 to the excitationsignal Ey in the Y-axis direction output from the excitation responseanalyzer 9 into the Y amplifier 11Y. Also, the addition signal ADDx thatnulls the excitation signal Ex in the X-axis direction is input into theX amplifier 11X. This makes it possible for the Y displacement sensor13Y (see FIG. 3) to measure the displacement of the rotating body in theY-axis direction.

The rotating body has the unbalance vibration in the Y-axis directioneliminated by the component of the oscillation signal Fy in the Y-axisdirection in the current supplied to the magnetic bearing 12, and the Ydisplacement sensor 13Y becomes able to measure the vibration amplitudeof the rotating body vibrated by the application of vibration.

As explained above, a configuration may be employed which independentlymeasure the response characteristics of the rotating body in the X-axisdirection and the response characteristics in the Y-axis direction.

As explained above, according to the vibration characteristic measuringapparatus 1 (see FIG. 1A) of this embodiment, when the responsecharacteristics of the rotating body (the rotational shaft 51 (see FIG.1A) and the centrifugal impeller 53 (see FIG. 1A)) of the multi-stagecentrifugal compressor 50 (see FIG. 1A) are measured, the oscillationsignals Fx and Fy generated by the balancing signal generator 8 (seeFIG. 3) are added to the excitation signals Ex and Ey generated by theexcitation response analyzer 9 (see FIG. 3) by the adder 7 a (see FIG.3), and the addition signal ADDx (see FIG. 3) containing the componentof the oscillation signal Fx in the X-axis direction and the componentof the excitation signal Ex in the X-axis direction, and the additionsignal ADDy (see FIG. 3) containing the component of the oscillationsignal Fy in the Y-axis direction and the component of the excitationsignal Ey in the Y-axis direction are generated.

The addition signals ADDx and ADDy are input into the current amplifier11 (the X amplifier 11X (see FIG. 3) and the Y amplifier 11Y (see FIG.3)) as the output signals by the excitation controller 7 (see FIG. 3).

The X amplifier 11X supplies the current obtained by amplifying theaddition signal ADDx into the magnets 12X (see FIG. 3) of the magneticbearing 12, and the Y amplifier 11Y supplies the current obtained byamplifying the addition signal ADDy into the magnets 12Y (see FIG. 3) ofthe magnetic bearing 12.

The rotating body has the unbalance vibration eliminated by thecomponent of the oscillation signal Fx in the X-axis direction containedin the current supplied to the magnets 12X (see FIG. 3), and thecomponent of the oscillation signal Fy in the Y-axis direction containedin the current supplied to the magnets 12Y (see FIG. 3). Also,vibrations are applied by the component of the excitation signal Ex inthe X-axis direction contained in the current supplied to the magnets12X and the component of the excitation signal Ey in the Y-axisdirection contained in the current supplied to the magnets 12Y.

According to this configuration, when the vibration characteristicmeasuring apparatus 1 (see FIG. 1A) measures the responsecharacteristics of the rotating body of the multi-stage centrifugalcompressor 50 (see FIG. 1A), it becomes possible to eliminate theunbalance vibration of the rotating body. This makes the allowable rangeof the amplitude of the forced vibration by the excitation expanded,thereby improving respective S/N ratios of the X displacement signal Mxand the Y displacement signal My output from the displacement sensor 13(the X displacement sensor 13X (see FIG. 3) and the Y displacementsensor 13Y (see FIG. 3)). Also, the excitation response analyzer 9 (seeFIG. 3) becomes able to obtain high-quality data on the displacement inthe X-axis direction and high-quality data on the displacement in theY-axis direction.

Furthermore, since the quality of obtained data is high and the responsecharacteristics of the rotating body can be evaluated with little data,the time necessary for obtaining pieces of data can be reduced. Also,the number of processes to obtain pieces of data can be reduced, andthus reducing the energy consumption.

The excitation signals Ex and Ey may be, for example, random signals andpulse signals instead of the above-explained sine wave signals.

When the excitation response analyzer 9 generates an excitation signalother than the sine wave signal, a configuration can be employed inwhich the rotating body control signal obtained by adding the componentof the excitation signal (the first direction excitation component andthe second direction excitation component) to the component (a cosinewave component and a sine wave component) of the oscillation signal isamplified by the current amplifier 11 (see FIG. 3) and the obtainedcurrent is supplied to the magnetic bearing 12 (see FIG. 3).

Also, in order to evaluate the stability of the rotating body, there isa technique for analyzing the vibration waveform of the rotating bodyand identifying a system parameter. For example, vibration is applied tothe rotating body by the excitation signal containing the sine wavecomponent of a frequency causing the rotating body to generate vibrationat a natural frequency, and the excitation signal is instantaneously cutoff when the rotating body is in a resonant condition to identify thesystem parameter based on a free vibration waveform (regarding thedetail of the system identification, see, for example, Shuichi ADACHI,“System Identification for a Control based on MATLAB”, 1996, Tokyo DenkiUniversity Press).

In the case of the technique for identifying the system parameter inthis fashion, a current obtained by amplifying, by the current amplifier11 (see FIG. 3), the rotating body control signal obtained by adding theexcitation signal which causes the rotating body to vibrate at a naturalfrequency to the oscillation signal can supplied to the magnetic bearing12 (see FIG. 3). Also, after the excitation signal is cut off, while therotating body is freely vibrating, when the currents obtained byamplifying the oscillation signals Fx and Fy by the current amplifier 11are supplied to the magnetic bearing 12, the displacement sensor 13 (seeFIG. 3) can measure the displacement of the rotating body freelyvibrating without being affected by the unbalance vibration.

As an example of the modified vibration characteristic measuringapparatus 1, for example, as shown in FIG. 6, the excitation controller7 having an excitation signal cutoff device 10 that cuts off respectiveoutputs of the addition signals ADDx and ADDy output from the adder 7 amay be employed.

The X displacement signal Mx output from the X displacement sensor 13Xand the Y displacement signal My output from the Y displacement sensor13Y are input into the excitation signal cutoff device 10.

The excitation signal cutoff device 10 is configured to cut off theoutput of the addition signals ADDx and ADDy when at least one of thedisplacement of the rotating body in the X-axis direction calculatedbased on the X displacement signal Mx and the displacement of therotating body in the Y-axis direction calculated based on the Ydisplacement signal My exceeds a predetermined threshold.

According to this configuration, for example, when the displacement inthe X-axis direction or the displacement in the Y-axis direction exceedsthe predetermined threshold by the excitation by respective componentsof the excitation signals Ex and Ey in the addition signals ADDx andADDy, i.e., the vibration amplitude of the rotating body exceeds theallowable value, the supply of the currents those are the additionsignals ADDx and ADDy amplified by the current amplifier 11 to themagnetic bearing 12 is cut off. Hence, the vibration the rotating bodycan be converged at the amplitude exceeding the allowable value, and forexample, it is possible to prevent the sealing mechanisms 55 (see FIG.1A) from being damaged.

It is not illustrated in the figure but if the excitation signal cutoffdevice 10 is disposed between the excitation response analyzer 9 and theadder 7 a, when the vibration amplitude of the rotating body exceeds theallowable value, the input of the excitation signals Ex and Ey outputfrom the excitation response analyzer 9 into the adder 7 a can be cutoff. Hence, only the oscillation signals Fx and Fy are output from theexcitation controller 7, and currents those are the oscillation signalsFx and Fy amplified by the current amplifier 11 are supplied to themagnetic bearing 12.

According to this configuration, the vibration at the amplitudeexceeding the allowable value and generated by the rotating body can befurther rapidly converged.

1. An apparatus for measuring vibration characteristics, comprising: amagnetic bearing that generates a magnetic force on a rotating body of arotating machine in a non-contact manner; a measuring device thatmeasures a vibration when the rotating body is vibrated; a currentsupplier that supplies a current to the magnetic bearing; and anexcitation controller which outputs an excitation control signal forcontrolling the magnetic bearing to apply vibration to the rotatingbody, and which measures a response characteristics of the vibration ofthe rotating body to the excitation control signal based on thevibration measured by the measuring device, wherein the excitationcontroller outputs a rotating body control signal obtained by adding avibration eliminating signal to the excitation control signal for themagnetic bearing to eliminate unbalance vibration generated when therotating body rotates when the response characteristics is measured, andthe current supplier supplies a current that generates magnetic force inaccordance with the rotating body control signal to the magneticbearing.
 2. The apparatus according to claim 1, wherein the excitationcontroller takes a cosine wave component at a same period as a rotationperiod of the rotating body as a component for eliminating vibration ofthe rotating body in a first direction orthogonal to a direction of anaxial line of the rotating body, generates the vibration eliminatingsignal containing a sine wave component at a same period as the rotationperiod of the rotating body as a component for eliminating vibration ofthe rotating body in a second direction orthogonal to the axial linedirection and the first direction, generates the excitation controlsignal containing a first direction excitation component for applyingvibration to the rotating body in the first direction and a seconddirection excitation component for applying vibration to the rotatingbody in the second direction, and adds the cosine wave component to thefirst direction excitation component, and adds the sine wave componentto the second direction excitation component to generate the rotatingbody control signal.
 3. The apparatus according to claim 1, wherein theexcitation controller comprises an excitation signal cutoff device thatcuts off an output of the rotating body control signal when thevibration amplitude measured by the measuring device exceeds apredetermined allowable value.
 4. The apparatus according to claim 2,wherein the excitation controller comprises an excitation signal cutoffdevice that cuts off an output of the rotating body control signal whenthe vibration amplitude measured by the measuring device exceeds apredetermined allowable value.
 5. A method for measuring vibrationcharacteristics executed by a vibration characteristic measuringapparatus, the apparatus including: a magnetic bearing that generates amagnetic force on a rotating body of a rotating machine in a non-contactmanner; a measuring device that measures an vibration when the rotatingbody is vibrated; and an excitation controller which outputs anexcitation control signal for controlling the magnetic bearing to applyvibration to the rotating body and a vibration eliminating signal foreliminating unbalance vibration generated when the rotating bodyrotates, and which measures a response characteristics of the vibrationof the rotating body to the excitation control signal based on thevibration measured by the measuring device, the method comprising stepsof: adding the excitation control signal to the vibration eliminatingsignal to generate a rotating body control signal; and measuring thevibration when a current that generates magnetic force in response tothe rotating body control signal is supplied to the magnetic bearing.